The present invention relates to the foaming or expansion of a synthetic resin composition and, more particularly, to foaming processes, foamable compositions and foamed articles involving an aliphatic olefin polymer, a blowing agent and from 0.1 to 10 percent by weight, based on said aliphatic olefin polymer, of at least one compound effective to suppress the escape of the blowing agent gas from the expanded foam, with a resultant improvement in the gas efficiency of the blowing agent together with a substantial reduction in shrinkage of the expanded foam.
Generally, thermoplastic synthetic resin foam is produced by adding a blowing agent to the resin and by heating the resultant resin/blowing agent mixture to gasify the blowing agent for expanding the resin. Improvement in the efficiency of such blowing agent is significant for commercial-scale production of synthetic thermoplastic resin foams.
Among thermoplastic resins, aliphatic olefin polymer resins are particularly difficult to expand directly in a commercial-scale process. This has been attributed to the high degree of temperature dependence of their thermo-rheological viscoelasticity in the molten state and the relatively high permeability of their foam cell membranes to blowing agent gases. Further complications arise from the latent heats of thermal expansion and crystallization which may occur during the foaming process. Commonly, especially for expansion ratios of five or more, aliphatic olefin polymers are modified prior to expansion by crosslinking or blending with other resin or resins.
As an alternative approach, Japanese Patent Publication No. 35(1960)-4,341 discloses extrusion-foaming of a non-crosslinked polyolefin resin using such a relatively expensive blowing agent as 1,2-dichlorotetrafluoroethane. However, the resulting foam product often undergoes a gradual shrinkage and a density change and has a poor surface appearance. Improved quality and dimensional control of olefin polymer foams are thus still desirable.
More recently, a variety of chemical additives have been discovered which improve the quality and dimensional stability of olefin polymer foams without the necessity of crosslinking and/or resin blending techniques while permitting the satisfactory utilization of less expensive volatile organic blowing agents in place of 1,2-dichlorotetrafluoroethane. More specifically, such additives include saturated higher fatty acid amides, saturated higher fatty acid amines and complete esters of saturated fatty acids as disclosed in Watanabe et al. U.S. Pat. No. 4,214,054; partial esters of long chain fatty acids as disclosed in Cronin U.S. Pat. No. 3,644,230 and in Ehrenfreund U.S. Pat. No. 3,755,208; partial and complete esters of aliphatic fatty acids as disclosed in Korori U.S. Pat. No. 4,217,319; and the related technology of Japanese Kakoi Nos. 53-102,971; 54-34,374; and 54-39,476.
The present invention provides another group of chemical additives which improve the dimensional stability of aliphatic olefin polymer foams. More specifically, the present invention provides a foamable olefin polymer composition which is easily expanded using commonly available inexpensive blowing agents to give a substantially closed-cell foam product exhibiting reduced shrinkage and improved dimensional stability and surface smoothness.